The present invention relates to a method for dual fuel injection into the combustion chamber of an internal combustion engine. More specifically, the present invention relates to a dual fuel injection method suitable for application in the internal combustion engine of a car, truck, bus, locomotive, ship or other engine-powered forms of transportation, as well as in stationary applications, such as engines related to energy production and industrial applications.
Due to the benefits of converting diesel-stroke engines so that they operate burning gaseous fuels has resulted in many recent developments in this area of engine technology. Many gaseous fuels, such as natural gas or hydrogen, by way of example, are clean burning fuels (relative to diesel), which means that when an engine substitutes such gaseous fuels for diesel fuel, the engine can, depending on a number of variables including the gaseous fuel chosen, operate with reduced emission levels of particulate matter (PM), hydrocarbons and nitrogen oxides (NOx).
A known method that allows diesel engines to operate using gaseous fuels utilizes a second fuel as well as the gaseous fuel. For the purposes of this discussion natural gas will constitute the gaseous fuel, however, other gaseous fuels such as hydrogen, methane, ethane, propane, lighter flammable hydrocarbon derivatives, etc. will also operate as gaseous fuels. Generally, natural gas is mixed with the intake air prior to the introduction of the air/natural gas mixture into the engine cylinder (a process known in the field involved here as fumigation). A homogeneous air/natural gas mixture is thus introduced into the piston cylinder during the intake stroke. During the compression stroke, the pressure and temperature of the homogeneous mixture is increased. Near the end of the compression stroke, a small quantity of pilot diesel fuel is employed to ignite the air/natural gas mixture. The advantage of employing a homogeneous mixture of air and gas is that the combustion fuel to air ratio (F/A ratio) can be controlled so as to burn in a lean homogeneous manner and achieve lower NOx emissions and lower particulate matter, compared to equivalent diesel-fuelled engines.
Note also that the homogeneous mixture of fuel and air can also be ignited by a spark or hot surface. Rather than the ignition of pilot fuel, a spark or hot surface can be employed to cause the homogeneous mixture to ignite, optimally near top dead center at the commencement of the power stroke.
This method of gaseous combustion, namely, employing a fumigated fuel, has a number of disadvantages. The first main disadvantage is encountered at high load engine operating conditions, when the elevated temperature and pressure in the piston cylinder during the compression stroke makes the air/natural gas mixture susceptible to xe2x80x9cexcessive knockingxe2x80x9d. Knocking is an uncontrolled combustion process resulting in a very high rate of heat release. Knocking is characterized, in most instances, by relatively rapid fluctuations in combustion chamber pressure. Excessive knocking is associated with conditions where the rapid heat release rate causes excessive combustion chamber pressure that is large enough to damage engine components. Excessive knocking can also cause engine damage through excessive heat release resulting in thermal damage to engine components, such as, by way of example, the piston crown.
A few measures for reducing the risk of excessive knocking include lowering the compression ratio of the engine or limiting the power and torque output, but these measures cause a corresponding reduction in the engine""s cycle efficiency (that is, not as much power is available from each piston stroke). A xe2x80x9cknock limitxe2x80x9d is designated and defined as that set of conditions within the cylinder at which excessive knock can occur as described above. Ultimately, fumigated fuel is knock limited and, as such, is unable to meet load demands beyond a certain level dictated by the knock limit.
For the purposes of this application, xe2x80x9cfumigated fuelxe2x80x9d or xe2x80x9cfumigated gaseous fuelxe2x80x9d will be a fuel (generally gaseous) and oxygen mix. Fumigated fuel is a fuel/oxygen mix where the fuel has been mixed with oxygen by the time the piston has reached top dead center immediately prior to both combustion and the piston power stroke. The oxygen would generally be provided as a constituent of intake air; however, it could be provided in some other manner. The fumigated fuel can be mixed with oxygen in the combustion chamber or in the intake manifold (or intake conduit) or partially mixed within both the intake manifold and combustion chamber. For the purposes of this discussion, the fumigated fuel will generally be substantially homogeneous, however, it can also be stratified to some extent.
The second main disadvantage of a homogeneous mixture of fuel and air is that, under low load engine operating conditions, the mixture of fuel and air becomes too lean to support stable combustion via flame propagation and results in incomplete combustion or misfiring. The intake air flow can be throttled to maintain a F/A ratio above a flammability limit, however, such throttling adversely affects the engine efficiency. As will be discussed further below, the flammability limit is defined as the condition limits with the engine under which the F/A ratio will begin to support a flame propagation combustion event.
Third, during start-up it is important that the fumigated fuel, introduced into the cylinder be ignited. However, as is well known in the field involved here, an initial injection of fumigated fuel and, in some cases, a pilot fuel, into the cylinder does not necessarily cause the engine to start on each attempt. When this occurs, a highly flammable mixture can flood the cylinder as well as the exhaust system. This could cause an uncontrolled combustion event when engine ignition is next attempted.
Recently, a different type of dual fuel combustion engine, herein referred to as a high pressure direct injection gas engine, has become known in the field involved here. Similar to the conventional dual fuel method described above, high pressure direct injection gas engines burn a large quantity of gaseous fuel, yielding an improvement over diesel-fuelled engines by reducing the emission levels of NOx and particulate matter. In addition, high pressure direct injection gas engines have been demonstrated to achieve the same combustion efficiency, power and torque output as state-of-the-art diesel-fuelled engines. The operational principle underlying high pressure direct injection gas engines is that two fuels are injected under pressure into the chamber near the end of the compression stroke. According to one method, a small quantity of xe2x80x9cpilot fuelxe2x80x9d (typically diesel) is injected into the cylinder immediately followed by a more substantial quantity of gaseous fuel. The pilot fuel readily ignites at the pressure and temperature within the cylinder at the end of the compression stroke, and the combustion of the pilot fuel initiates the combustion of the gaseous fuel that might otherwise be difficult to ignite. Known high pressure direct injection gas engines have no fumigated fuel. As a result, the directly injected gas operates in a xe2x80x9cdiffusion combustionxe2x80x9d mode, rather than a premixed combustion mode. In a diffusion combustion mode typical of homogeneous fuel combustion, the bulk of the combustion is believed to occur in a local near-stoichiometric reaction zone, where the temperature and resulting NOx formation are relatively high (compared to the temperature and resulting NOx formation caused by a lean burn premixed combustion).
In U.S. Pat. No. 5,365,902 (hereinafter referred to as the ""902 patent), a method and apparatus for dual fuel injection is disclosed, which combines some of the advantages of diffusion combustion and premixed combustion with flame propagation. According to the ""902 patent, the engine load conditions are detected, and under low load conditions, the pilot fuel is injected into the cylinder prior to the injection of the gaseous fuel. When the directly injected gaseous fuel is introduced after the pilot fuel is injected, the directly injected gaseous fuel burns in a diffusion mode shortly after entering the combustion chamber. Alternatively, under high load conditions, the gaseous fuel is injected into the combustion chamber prior to the injection of the pilot fuel. In this manner, the gaseous fuel that is injected prior to the introduction of the pilot fuel will mix with air in the combustion chamber, so as to form a homogeneous mixture, which burns by way of flame propagation.
A drawback of the dual fuel injection technique disclosed in the ""902 patent is the risk of excessive knocking under high load conditions. As the load is increased, the required fuel to air ratio of the early-injected gaseous fuel is increased. When the F/A ratio is high, there is a risk that compression (and the resulting increase in temperature and pressure) of the early-injected gaseous fuel will cause it to ignite prematurely resulting in excessive knocking. This limit on the quantity of early-injection gaseous fuel is referred to herein as the xe2x80x9cknock limitxe2x80x9d, at or above which, excessive knocking will occur as described above. Because the device disclosed in the ""902 patent is knock limited, it can not be employed for engines which target a high power density, nor can it operate as efficiently under high load conditions. As mentioned above, excessive knocking can also cause damage to the engine, reduce engine durability, limit the range of gaseous fuel quality or fuel composition that can be employed, and limit the engine""s power output.
U.S. Pat. No. 5,329,908 (hereinafter referred to as the ""908 patent) discloses a method of using high pressure direct injection of gaseous fuel. According to the method taught by the ""908 patent, gaseous fuel is injected at high pressure, when the piston is at or near top dead center at the end of the compression stroke, thereby providing diesel-like engine cycle efficiencies. When the pressure of the gaseous fuel reservoir drops below about 2,000 pounds per square inch, a controller changes the injection mode, causing the gaseous fuel to be introduced to the cylinder much earlier. For example, the gaseous fuel can be injected during the intake stroke (that is, when air is being drawn into the combustion chamber). In this manner, the gaseous fuel is allowed to mix with the air in the combustion chamber, forming a homogeneous mixture that will burn by supporting flame propagation. The method disclosed in the ""908 patent requires a spark or glow plug to ignite the gaseous fuel in the combustion chamber.
The method disclosed in the ""908 patent has several disadvantages, however. When operating in its early injection mode (that is, below 2000 pounds per square inch), it is subject to excessive knocking, as discussed above, limiting engine efficiency and power density. Also, under low load conditions, the engine will reach a limit where combustion of the early-injected gaseous fuel does not proceed satisfactorily (poor combustion quality), because the F/A ratio of the mixture is too low (that is, too lean) to support flame propagation. As noted above, this low load limit, resulting from a low F/A ratio, is referred to herein as the xe2x80x9cflammability limitxe2x80x9d. The method disclosed in the ""908 patent also relies on a glow plug or spark plug, yielding different combustion characteristics than fuel ignited by pilot diesel. In general, conventional systems that employ glow plugs or spark plugs require a relatively rich air-fuel mixture (compared to ignition by a pilot fuel) to be formed prior to ignition, which results in higher heat release rates and relatively more NOx formation.
U.S. Pat. No. 5,711,270 (hereinafter referred to as the ""270 patent) discloses a technique for high pressure injection of both oil and gaseous-based fuel. The timing and the quantity of gaseous fuel injection is varied when the engine is under different load conditions, and the ignition of the fuel is commenced with the introduction of the oil based fuel (that is, the pilot fuel) into the combustion chamber. The ""270 patent discloses one method of implementing the aforementioned high pressure process. Because the ""270 patent discloses invariably injecting the pilot fuel first, to initiate combustion, followed by the injection of the gaseous fuel, this method shares the same major disadvantages as other known high pressure direct injection methods, which include invariably burning in a diffusion mode of combustion and not being able to take advantage of a lean premixed combustion, which can yield lower NOx and particulate matter emissions.
The present fuel injection technique provides an improved method for the injection of fuel into the combustion chamber of an internal combustion engine that avoids the problem noted above.
At the same time, the present fuel injection technique provides a method of mono and dual fuel injection into the combustion chamber of an internal combustion engine which combines the advantages of lean premixed combustion over a range of operational conditions with some of the advantages of diffusion combustion in low and high load operational conditions while avoiding problems associated such a method. These problems arise where limited directly injected fuel is employed and where limited fumigated fuel is employed.
The amount of fumigated fuel is generally controlled by the amount of fuel introduced for fumigation by a variable control valve. During operation, there is some leakage across the variable control valve when the fuel stop valve is open. Therefore, where a flow rate at or below the leakage flow rate is required, control over the fumigation flow can be compromised where that flow is determined by the leakage rate across the variable control valve. While this problem flows from the apparatus generally employed to control fumigation and, therefore, fumigated fuel combustion, it does not generally arise where fumigated fuels alone are employed to drive a piston. That is, stable combustion is difficult to achieve at a fumigation flow rate at or below the leakage flow rate. As will be discussed below, however, this problem is an important consideration if fumigated fuel is employed in conjunction with other combustion techniques such as high pressure direct injection of gaseous fuels.
Moreover, high pressure direct injection requires injector tip heat management as the injector tip is disposed in the combustion chamber. Injector operation can be affected by heat if no fuel or a limited amount of fuel is consistently provided through the injector for an extended number of combustion events. This can occur when combining a second combustion mode to drive the engine pistons.
The present fuel injection technique addresses or manages these concerns associated with combined directly injected fuel and fumigated fuel.
The present fuel injection technique also provides a method of fuel injection into the combustion chamber of an internal combustion engine that compensates for problems known to engines that employ conventional fumigation during start-up or low speed low load conditions when undesirable flammable mixtures are injected into the cylinder and, initially, fail to ignite.
The present fuel injection technique further provides a set of operating parameters for fuel injection into the combustion chamber of an internal combustion engine which avoids excessive knock associated with high load conditions as seen in prior art noted above.
The present method and apparatus overcome the shortcomings noted above by introducing fuel into a combustion chamber of an internal combustion engine having at least one cylinder with a piston. The cylinder and piston partially defining the combustion chamber where the piston oscillates within the cylinder between top dead center and bottom dead center when the internal combustion engine is operating. The method comprises selecting either a low load operating mode or a high load operating mode. The operating modes are distinguished by the combustion characteristics of a fumigated gaseous fuel within the combustion chamber. The low load operating mode is defined where the fumigated gaseous fuel is unable to support stable premixed combustion. The high load operating mode is defined where the fumigated gaseous fuel is able to support stable premixed combustion. In the high load operating mode, the fumigated gaseous fuel is introduced into the combustion chamber before the piston is at top dead center. In the low load operating mode, if fumigated gaseous fuel is introduced, realizing that some may not be introduced during start-up, providing that fumigated gaseous fuel into the combustion chamber before the piston is at top dead center. In the low load operating mode, a quantity of a second gaseous fuel is directly injecting into the combustion chamber when the piston is at or near top dead center. The fumigated gaseous fuel and the second gaseous fuel are ignited within the combustion chamber when the piston is at or near top dead center.
A further embodiment of the present method and apparatus includes, in the high load operating mode, directly injecting a quantity of the second gaseous fuel into the combustion chamber when the piston is at or near top dead center. The second gaseous fuel and the fuel in the fumigated fuel can be the same.
In the high load operating mode, the fumigated gaseous fuel can be ignited by way of homogeneous charge compression ignition.
Also, the second gaseous fuel can be ignited as a result of ignition of a pilot fuel that is more auto-ignitable than the second gaseous fuel. In the high load operating mode, ignition of the fumigated gaseous fuel can be as a result of ignition of a pilot fuel that is more auto-ignitable than the fumigated gaseous fuel. In either case the pilot fuel can be directly injected into the combustion chamber.
In the low load operating mode, the pilot fuel is directly injected into the combustion chamber when the piston is at or near top dead center. In the high load operating mode, the pilot fuel can be directly injected into the combustion chamber during an intake stroke or compression stroke of the piston or when the piston is at or near bottom dead center. A preferred embodiment has injection effected between 120 and 60 degrees prior to top dead center as measured in degrees of crankshaft rotation. The pilot fuel can also be injected in two stages, a first stage during an intake stroke or compression stroke of the piston or when the piston is near bottom dead center or when the piston is between 120 and 60 degrees prior to top dead center as measured in degrees of crankshaft rotation. A second stage injection of pilot fuel is included when the piston is at or near top dead center prior to or at the commencement of the power stroke.
A further embodiment the method includes igniting the fumigated gaseous fuel and the second gaseous fuel by way of a hot surface, including a glow plug, or a spark.
A further embodiment includes in the low load operating mode, employing a low speed low load operating mode and a high speed low load operating mode. The low speed low load operating mode defined by an engine speed range, as measured in crankshaft revolutions per minute (RPM), from and including zero up to a pre-defined operating speed. The high speed low load operating mode is defined by an engine speed at or above the pre-defined operating speed. In the low speed low load operating mode, barring the fuel in the fumigated gaseous fuel from the combustion chamber thereby eliminating fumigated fuel from the combustion chamber. In the high speed low load operating mode, a flow of the fuel in the fumigated gaseous fuel is provided in excess of a pre-defined leakage flow rate.
In the above embodiment, the flow of the fuel in the fumigated gaseous fuel is regulated by at least one of a shut-off valve and a variable control valve. Each of these components is disposed within a fuel passage defined by a fuel conduit. The fuel passage is in communication with an intake passage defined by an intake conduit such that the fuel passage and the intake passage direct the fuel in the fumigated gaseous fuel into the combustion chamber during an intake stroke.
A further embodiment includes the method where the shut-off valve is closed in the low speed low load operating mode prohibiting the fuel in the fumigated gaseous fuel from entering the combustion chamber.
The fuel in the fumigated gaseous fuel can also be directly injected into the combustion chamber and, therefore, substantially mixed with oxygen in the combustion chamber.
The method includes a fumigated gaseous fuel at a quantity at or below a pre-defined knock limit when the high load operating mode is employed. The quantity of the second gaseous fuel can be zero or a pre-defined injector tip maintenance limit when combustion of the fumigated gaseous fuel meets engine load requirements. If load is not met by fumigated gaseous fuel combustion, the quantity of the second gaseous fuel is determined by engine load requirements beyond that met by combustion of the fumigated gaseous fuel.
An apparatus is also disclosed that introduces fuel into a combustion chamber of an internal combustion engine having at least one cylinder with a piston, the cylinder and the piston partially defining the combustion chamber. The apparatus includes measuring devices for collecting operational data from the engine such as engine speed, engine load demand and excessive knock information. A controller is also included that is capable of processing the operational data to create an engine profile, and, with that information, directing the engine to operate in one of a low load operating mode and a high load operating mode based on the engine profile. The low load operating mode and the high load operating mode are distinguished by combustion characteristics of a fumigated gaseous fuel within the combustion chamber. In the low load operating mode the fumigated gaseous fuel is unable to support stable premixed combustion. In the high load operating mode the fumigated gaseous fuel is able to support stable premixed combustion. The apparatus further includes a main fuel injector capable of directly injecting a second gaseous fuel into the combustion chamber. A pilot fuel injector is provided that is capable of injecting a pilot fuel into the combustion chamber. The pilot fuel is more auto-ignitable than the fumigated gaseous fuel or the second gaseous fuel. Finally, an intake conduit is provided for directing the fumigated gaseous fuel into the combustion chamber. In the low load operating mode, the control unit then directs, if fumigated gaseous fuel is to be provided, the fumigated gaseous fuel through the intake conduit making it available within the combustion chamber before the piston is at top dead center. The main fuel injector is directed, in the same operating mode, to introduce the second gaseous fuel into the combustion chamber when the piston is at or near top dead center. Finally, in the low load operating mode, the pilot fuel injector introduces the pilot fuel into the combustion chamber when the piston is at or near top dead center. In the high load operating mode, the control unit directs the fumigated gaseous fuel through the intake conduit to be available in the combustion chamber when the piston is at or near top dead center. The fumigated fuel in this mode falls below a pre-defined knock limit. The pilot fuel injector introduces pilot fuel into the combustion chamber during the compression stoke of the engine when operating.
The apparatus control unit can further direct the main fuel injector to introduce the second gaseous fuel into the combustion chamber when the piston is at or near top dead center.
Finally, the control unit in the apparatus, in the low load operating mode, can direct the engine to operate in either a low speed low load operating mode or a high speed low load operating mode based on the engine profile. The low load operating modes are distinguished by engine speed as measured in crankshaft revolutions per minute (RPM). In low speed low load operating mode the engine speed range is from and including zero up to a pre-defined operating speed. In the high speed low load operating mode, the engine speed is at or above the same pre-defined operating speed. In low speed low load operating mode, the control unit prohibits the fumigated gaseous fuel from the combustion chamber. In the high speed low load operating mode, the control unit directs the fuel in the fumigated gaseous fuel through to the intake conduit in a quantity in excess of a pre-defined leakage flow rate.
The present fuel injection technique provides a method of mono- and dual-fuel injection into the combustion chamber of an internal combustion engine, which combines the advantages of lean fumigated fuel combustion over a range of operational conditions with some of the advantages of diffusion combustion in low and high load operational conditions.
The present fuel injection technique provides for a fuel injection strategy that provides more reliable start-up and, at the same time, is able to operate taking advantage of fumigated fuel combustion characteristics once the engine is started and operating under load.
The present fuel injection technique provides a set of operating parameters for fuel injection into the combustion chamber of an internal combustion engine, which utilizes fumigated fuel to benefit from lean burn homogeneous combustion under higher loads of operation while avoiding excessive knock associated with high load conditions.
The present fuel injection technique provides a method of fuel injection into the combustion chamber of an internal combustion engine, which retains the high efficiency and high cycle output of high pressure direct injection, retains the advantage of lower NOx and particulate matter emissions normally associated with lean burn combustion of fumigated fuel, and helps maximize power density while avoiding excessive knocking.
The present fuel injection technique finally provides a method of introducing fuel into a combustion chamber of an operating internal combustion engine having at least one cylinder with a piston, said engine having a low load mode of operation and a high load mode of operation, and wherein said fuel comprises a main fuel and a pilot fuel that is more auto-ignitable than said main fuel, said method comprising:
(a) detecting a set of load conditions on said engine; and
(b) employing said low load operating mode when a first predetermined set of load conditions is detected, said first predetermined set of load conditions corresponding to load conditions that exist when the desired ratio of said main fuel to air is less than a calibrated premixed combustion stability limit of a homogeneous mixture of said main fuel and intake air, and employing said high load operating mode when a second predetermined set of load conditions is detected, wherein at an operating engine speed as measured in the form of crankshaft revolutions per minute, said second set of load conditions corresponds to an engine load that is greater than the engine load corresponding to said first predetermined set of load conditions.